1. Field of the Invention
The invention relates generally to the art of extracting titanium from titaniferous materials. In particular, the invention relates to the use of thionyl chloride to convert the titanium contained in such materials to titanium tetrachloride and also to the removal of the titanium chloride from the reacted materials.
2. Prior Art
The extraction of titanium from titanium-rich ores and minerals is an important industrial process used to produce titanium metal and metal alloys, titanium tetrachloride, titanium dioxide for use as a pigment and opacifier, and other titanium compounds. The principal ores and minerals employed are rutile, ilmenite and leucoxene. Two proceses are used commercially: in one, known as the "sulfate" process, ilmenite is dissolved in sulfuric acid, ferrous sulfate is partially separated by crystallization and hydrous titanium dioxide is then precipitated by hydrolysis of the ferruginous titanium sulfate solution. In the other process, known as the "chloride" process, rutile or mixtures of rutile, ilmenite and leucoxene are mixed with coke and chlorinated to volatilize titanium tetrachloride which is purified by distillation and then oxidized at a high temperature to produce titanium dioxide and regenerate chlorine.
The principal disadvantage of the sulfate process is that it generates large volumes of ferrous sulfate and dilute sulfuric acid containing iron as waste materials which are difficult to dispose of and, for this reason, it is now generally preferred to use the chloride process which generates a much smaller amount of objectionable waste material. However, the chloride process requires that the chlorination be carried out at a temperature of about 900.degree. C. at which chlorine and the chloride reaction products have a highly corrosive effect on the refractory structure of the chlorinator; also high energy input is required. Moreover, the chloride process requires a feed-stock which is principally composed of rutile of which commercially significant deposits exist in only a few limited areas of the world and the continually increasing demand for it to produce titanium products has frequently led to shortage of supply. Alternative sources of titaniferous feedstock suitable for the chloride process have therefore been sought: among these is the so-called "synthetic rutile" or "up-graded ilmenite" produced by an elaborate and expensive process in which iron is selectively extracted from the more abundant mineral ilmenite. Another substitute for natural rutile is the so-called "Sorel slag" obtained by electric furnace reduction of ilmenite to produce molten iron and a titaniferous slag (Sorel slag). This slag can be used for the extraction of titanium either by the sulfate or by the chloride process, but its availability is generally limited by economic factors relative to the production of iron by this electro-reduction process since it is usually much cheaper to produce iron by the customary blast-furnace process using cheap, abundantly available iron ores. There is, thus, a need for other, more widely available, sources of titaniferous feed-stock for the titanium industry.
Titanium is widely distributed throughout the world in association in various mineralogical forms with other, more abundantly occurring minerals or ores. For examples, kaolins commonly contain 1 to 2% TiO.sub.2, and bauxites contain 1 to 6% TiO.sub.2. Processes have been developed in which titanium is extracted as the tetrachloride from clays and bauxite, but these invariably involve chlorination at temperatures in the region of 900.degree. C. at which the mineral itself is consumed in the reaction. Usually the primary objective has been to produce aluminum trichloride with titanium tetrachloride as a by-product. One such process is the "Toth Process" described in the following U.S. Pat. Nos. 3,615,359, 3,615,360, 3,677,742, 3,713,809 and 3,713,811.
For some commercial applications of minerals such as kaolins or other clays and bauxites, high levels of TiO.sub.2 are disadvantageous and the removal of some or all of the titanium is a desirable objective. Hereto, however, no process has been developed whereby the titanium can be extracted in a form suitable for making titanium metal, titanium dioxide or other titanium products without the parent mineral itself being substantially consumed at the same time.
It has long been known that pure titanium dioxide can be converted to titanium tetrachloride by reaction with thionyl chloride at comparatively low temperatures. The reaction was first reported by G. Darzens and F. Bourion (Comptes rendus hebdomaires des seances de 1'Academie des Sciences, Vol. 153, pages 1270-1272 (1911). A more accurate description of the reaction was given by H. Hecht, G. Jander and H. Schlapmann (Zeitschrift fur Anorganischen Chemie, Vol. 254, pages 255-264 (1947). As described in the latter publication, titanium dioxide, mixed with an excess of thionyl chloride, was heated for several hours at 300.degree. C. in a sealed tube and the excess SOCl.sub.2 was separated by distillation from the TiCl.sub.4 produced in the reaction. No record has been found of the reaction of thionyl chloride with titanium when this is a constituent of other materials. To the best of our knowledge the prior art is devoid of a teaching or even a suggestion for the use of thionyl chloride to extract or remove titanium on a selective basis from titaniferous ores or minerals.
Accordingly, an objective of our invention is to provide an improved process for extracting titanium from titaniferous minerals and ores. Another objective is to provide a process whereby titanium can be recovered as tetrachloride from titaniferous minerals and ores at a reaction temperature substantially lower than that employed in the chlorination process hereto employed. Another objective is to provide a process whereby titanium can be extracted, wholly or in part, from minerals or ores in which it is a minor constituent without destroying the parent mineral.